Generally, a magnetic recording medium comprises a substrate comprising of, for example, a polyester film and magnetic layer formed on the substrate by the application of a magnetic coating composition containing a magnetic powder and a binder resin on the surface of the substrate and drying the coated composition. In order to impart good electromagnetic conversion characteristics to the magnetic recording medium, it is necessary to improve the surface smoothness of the magnetic layer and also to increase the density of the magnetic powder in the layer. However, the surface smoothness and the density of the magnetic powder are influenced by the dispersibility and packing properties of the magnetic powder in the binder resin.
In addition, since the magnetic recording medium contacts objects at a very high rate including, e.g., a magnetic head, during recording and reproducing, the magnetic layer is required to have good durability.
Hitherto, various binder resins have been used for the magnetic recording medium including vinyl chloride/vinyl acetate type copolymers, poluyrethane resins, cellulose type resins, polyvinyl butyral resins, polyester resins, and the like. Among them, the vinyl chloride/vinyl acetate/vinyl alcohol terpolymer is preferably used due to the good dispersibility and packing properties of the magnetic powder therein.
Recently, improvement of the recording characteristics of a magnetic recording medium particularly in the short wavelength reange is made to satisfy the strong desire for high density recording. To this end, it is necessary to further improve the dispersibility of the magnetic powder in the binder resin to increase the surface smoothness and durability of the magnetic layer. However, none of the conventional binder resins satisfy this requirement.
It was also revealed that the dispersibility and packing properties as well as orientation of the magnetic powder in the binder resin are improved by incorporation of a hydrophilic polar group such as a carboxy group, a sulfonic acid group and a phosphoric acid group in the binder resin. Since the surface of the magnetic powder particle is generally hydrophilic due to the presence of highly polar hydroxyl groups or absorbed water, the affinity of the resin for an organic solvent due to inherent hydrophobic property of the resin and that for the magnetic powder due to the incorporated polar graoup can be well compromised so that above impovements are achieved (cf. The Journal of the Japanese Adhesive Association, Vol. 17, (1981) 155. It is also proposed to improve the dispersibility etc. of the magnetic powder in the binder resin by using, as the binder resin, a radiation curable resin having the above polar groups (cf. Japanese Patent Kokai Publication (unexamined) No. 79427/1984).
Although the incorporation of the polar group in the binder resin can improve the dispersibility and packing properties of the magnetic powder therein to some extent, the degree of the improvement greatly varies with the kind of the binder resin and the condition of incorporation of the polar group. Therefore, the polar group containing binder resin is not satisfactory and should be improved.
Japanese Patent Kokai Publication (unexamined) No. 20315/1985 discloses a modified polyurethane used as a binder resin for the magnetic layer, prepared by synthesizing a polyesterpolyol, incorporating a phosphorous compound of the formula: ##STR2## wherein R.sub.1 is a hydrocarbon group or an ester-forming group such as a hydroxyl group, R.sub.2 is a hydrocarbon group, R.sub.3 is a hydrogen atom, a hydrocarbon group, an alkali metal atom or an ester-forming group such as a hydroxyl group, M is an alkali metal atom, and n is 1 or 2 to said polyesterpolyol through an ester bond, and then reacting the phosphorus compound incorporated synthesized compound with a polyisocyanate compound. However, such modified polyurethane does not satisfactorily improve the dispersibility of the magnetic powder in the binder resin.